This invention relates generally to gas turbine engines, and more particularly, to methods of depositing protective coatings on components of gas turbine engines.
Gas turbine engines typically include high and low pressure compressors, a combustor, and at least one turbine. The compressors compress air which is mixed with fuel and channeled to the combustor. The mixture is then ignited for generating hot combustion gases, and the combustion gases are channeled to the turbine which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to provide turbine, combustor and augmentor components with an environmental coating that inhibits oxidation and hot corrosion, or a thermal barrier coating (TBC) system that, in addition to inhibiting oxidation and hot corrosion, also thermally insulates the component surface from its operating environment.
Coating materials that have found wide use as environmental coatings include diffusion aluminide coatings, which are generally single-layer oxidation-resistant layers formed by a diffusion process, such as pack cementation. Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition to form two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. Beneath the additive layer is a diffusion zone that includes various intermetallic and metastable phases that form during the coating reaction as a result of diffusion gradients and changes in elemental solubility in the local region of the substrate. During high temperature exposure in air, the MAl intermetallic forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the diffusion coating and the underlying substrate.
High reliability TBC bond coats consisting of a NiAl overlay coating is highly sensitive to aluminide processing. Aluminide before and /or after the NiAl coating can result in substantial degradation of the TBC cyclic life. However, in order to protect the inside cooling passages from oxidation and hot corrosion, a vapor phase aluminide is required. This cross-functional requirement between external and internal surfaces of a turbine blade forces a highly labor intensive and costly process of vapor phase aluminiding (VPA) coating, chemical stripping of aluminide from external and protecting the internal passages while chemical processing. Additionally, these steps add the risk of chemically attacking the coating deposited on the internal passages.
Known process technology consists of VPA coating, at about 1800° F. to about 2000° F., the entire blade including both internal and external surfaces, filling inside passages with wax to protect from chemical attack, striping Al from the external surfaces by chemical surface treatment, removing the wax, and heat tint to assure that all aluminide is removed. These process steps can add a cost penalty and about 7-10 days of added manufacturing time.